This invention relates in general to the field of electronics and more particularly to a circuit for providing built-in frequency testing of a frequency generating circuit.
Electronic circuits such as low current crystal oscillators which produce an output frequency signal are used in numerous types of electronic devices. The output signal produced by an oscillator circuit can be used for example as a reference clock signal in an integrated circuit (IC) or other type of electronic circuitry. During the manufacture of an IC or other device incorporating such an oscillator circuit, it is beneficial to test the oscillator circuit to determine if the circuit is operational and producing the required output frequency signal. In some applications (e.g., time keeping circuits, etc.), it may be beneficial to monitor the performance of the frequency generating circuit (e.g., oscillator circuit) on a periodic basis in order to guarantee that the frequency has not changed. This periodic testing is highly beneficial in multi-stage frequency generating circuits in which anyone of the multiple stages can go bad. Such testing helps guarantee that the oscillator circuit is functioning properly.
Prior art approaches for testing such frequency generating circuits although helpful have either required too long to perform or have been limited by the working temperature range requirements of the IC or circuitry under test. Therefore, a need exists for a method and apparatus which can provide a frequency test which can overcome the above mentioned problems.
Briefly, in accordance with one embodiment of the invention, a built-in frequency test circuit includes a reference signal circuit which produces a reference signal having substantially the same frequency as the output frequency signal generated by a frequency generating circuit when it is operational. The test circuit further includes a comparison circuit for comparing the frequency of the output frequency signal generated by the frequency generating circuit and the frequency of the reference signal. The test circuit provides a test condition signal which indicates the operational condition of the frequency generating circuit.
In a second embodiment of the invention, the built-in frequency test circuit can be selectively placed between a test mode in which the test circuit is operational and a non-test mode in which the test circuit is not operational. In a further embodiment, the built-in test circuit is located within an IC.